Standard nucleic acid separation techniques limit researchers' abilities to analyze samples for nucleic acids that are present in low abundance, such as mutations. In particular, it is difficult to resolve rare nucleic acids which are present at low concentrations in the presence of closely-related nucleic acids, e.g., wild-type DNA. To overcome this problem, typically all of the nucleic acids in a sample are amplified prior to isolation and analysis. For example, using Polymerase Chain Reaction (PCR) amplification, each nucleic acid can be amplified one million times (or more). Theoretically, there will be a million-fold increase of each nucleic acid originally present, and, thus, a greater opportunity to isolate and find the nucleic acids in low abundance.
In practice, however, PCR amplification has significant drawbacks when it is used to analyze nucleic acids that are present in low abundance. The PCR reaction is stochastic and to the extent that a low-abundance nucleic acid is not amplified in the first few rounds of PCR, it likely will not be detected. In addition, PCR amplification introduces sequence errors in the amplicons. If the error rate is high enough, there can be a significant effect on the resulting sequence data, especially in applications requiring the detection of rare sequence variants.
Unfortunately, a viable alternative to sequencing plus amplification does not exist. Commonly-available separation techniques do not have the resolution or fidelity to pull enough low-abundance nucleic acids from a background to be useful. In fact, most separation methods require an amplification step, either before or after separation, to recover enough low-abundance nucleic acid for further analysis, e.g. sequencing.
For many years, nucleic acids have been separated from each other using electrophoresis. Electrophoresis involves directing the movement of charged particles in a medium, such as a gel or liquid solution by applying an electric field across the medium. The electric field may be generated by applying a potential across electrodes that are placed in contact with the medium such that electric current can be conducted into the medium. The movement of the particles in the medium is affected by the magnitude and direction of the electric field, the electrophoretic mobility of the particles and the mechanical properties, such as viscosity, of the medium. Through electrophoresis, particles that are distributed in a medium can be transported through the medium. Electrophoresis is commonly used to transport nucleic acids (such as DNA or RNA) through gel substrates. Because different species have different electrophoretic mobilities, electrophoresis may be used to separate different species from one another. Conventional electrophoresis techniques are largely limited in application to the linear separation of charged particles. Using conventional electrophoresis techniques, a direct current (DC) electric field or an alternating pulsed-field electrophoretic (PFGE) field is typically applied to a medium so that particles in the medium are transported toward an electrode.
Electrophoresis may be used to transport fragments of DNA or other microscopic electrically charged particles. Various electrophoresis methods are described in Slater, G. W. et al. Electrophoresis 2000, 21, 3873-3887. Electrophoretic particle transport is typically performed in one dimension by applying a direct current (DC) electric field between electrodes on either side of a suitable electrophoresis gel. The electric field causes electrically charged particles in the gel to move toward one of the electrodes. Because the particles experience different mobilities through the gel due to the DC field, the particles can be separated. In an alternate application, an asymmetric alternating current (AC) waveform can cause net drift of electrophoretic particles due to nonlinearity of the relationship between particle speed and applied electric field. This effect can be used to cause particles to move in one dimension as described in Chacron, M. J., et al. Phys. Rev. E 1997, 56, 3446-3450; Frumin, L. L, et al. Phys. Chem. Commun. 2000, 11; and, Frumin, L. L. et al. Phys. Rev. E 2001, 64, 021902. Additionally, Pohl, H. A., Dielectrophoresis: The Behavior of Neutral Matter in Nonuniform Electric Fields Cambridge University Press, Cambridge, UK 1978; Asbury, C. L., et al., Electrophoresis 2002, 23, 2658-2666; and Asbury, C. L., et al. Biophys. J. 1998, 74, 1024-1030 discloses that dielectrophoresis can be applied to concentrate DNA in two or more dimensions. However, applications of dielectrophoresis have required undesirably high electric field gradients, resulting in breakdown of the separation media or the samples. Each of these references is incorporated herein in its entirety.
Electrophoresis can also be used to concentrate particles in a particular location. A problem that can interfere with the successful use of electrophoresis for concentrating particles is that there must be an electrode at the location where the particles are to be concentrated. Electrochemical interactions between the electrodes and particles can degrade certain kinds of the particles. For example, where the particles comprise DNA, the DNA can be damaged by electrochemical interactions at the electrodes. Additionally, electric fields present during conventional direct current electrophoresis are divergence-free everywhere except at electrodes which can source or sink electric current. Thus, electrophoresis is typically applied in cases where particles are caused to move toward an electrode. Once concentrated, the particles can be obtained by cutting out the portion of the medium in which the particles have been concentrated. The particles can then be separated from the medium by using various purification techniques.
Additional methods of nucleic acid separation are known. References that describe methods for DNA separation include: Slater et al. The theory of DNA separation by capillary electrophoresis Current Opinion in Biotechnology 2003 14:58-64; Slater et al. U.S. Pat. No. 6,146,511 issued 14 Nov. 2000; Frunin et al. Nonlinear focusing of DNA macromolecules Phys. Rev. E 64:021902; Griess et al. Cyclic capillary electrophoresis Electrophoresis 2002, 23, 2610-2617 Wiley-VCH Verlag GmbH & Co. Weinheim (2002). References which describe the use of fields to separate particles include: Bader et al. U.S. Pat. No. 5,938,904 issued on Aug. 17, 1999; Bader et al. U.S. Pat. No. 6,193,866 issued on 27 Feb. 2001; Tessier et al: Strategies for the separation of polyelectrolytes based on non-linear dynamics and entropic ratchets in a simple microfluidic device Appl. Phys. A 75, 285-291 (2002); Chacron et al. Particle trapping and self-focusing in temporally asymmetric ratchets with strong field gradients Phys. Rev. B 56:3 3446-3550 (September 1997); Dean et al. Fluctuation driven ratchets: molecular motors Phys. Rev. Lett. 72:11 1766-1769 (14 Mar. 1994); Bier et al. Biasing Brownian motion in different directions in a 3-state fluctuating potential and an application for the separation of small particles Phys. Rev. Lett. 76:22 4277-4280 (27 May 1996); Magnasco, Forced thermal ratchets Phys. Rev. Lett. 71:10 1477-1481 (6 Sep. 1993). Each of these references is incorporated herein in its entirety.
There is a need for techniques that provide for high fidelity enrichment of nucleic acids without introducing errors into the sample and with an ability to isolate rare nucleic acids in a sample and to resolve nucleic acids having similar sequences.